A number of methods are conventionally known as methods for electrophotography. In general, fixed images are obtained by uniformly electrostatically charging by, e.g., corona charging or by direct charging using a charging roller the surface of a latent image bearing member comprising a photoconductive material, forming an electrostatic latent image on the latent image bearing member by its irradiation with light energy, subsequently developing this electrostatic latent image by the use of a toner standing charged positively or negatively, to form a toner image, and transferring the toner image to a transfer material such as paper as occasion calls, and thereafter fixing the toner image to the transfer material by the action of heat and/or pressure. The toner that has not been transferred to and has remained on the photosensitive member at the time of transfer is removed by various means, and then the above process is repeated.
Developing systems by which this electrostatic latent image is rendered visible include a dry-process developing method and a wet-process developing method as classified roughly. The dry-process developing method is divided into a method making use of a two-component developer constituted of a toner and a carrier, and a one-component developer constituted of only a toner.
In recent years, in image forming apparatus which employs electrophotography, such as printers or copying machines, they are required to form images having a higher resolution, in circumstances where they are variously demanded to be made compact and light-weight, high-speed and high-productivity, energy savable, highly reliable, low-cost, maintenance-free and so forth. In particular, images formed by using a black toner are required to be very minutely and faithfully reproduced up to full details in not only black and white image forming apparatus but also color image forming apparatus. Accordingly, as toners, they are required to be more improved in developing stability and charging stability.
As toners used in these image forming apparatus, they are commonly prepared by forming toner particles (toner base particles) comprising a thermoplastic resin and dispersed therein a colorant such as a dye or a pigment, a wax as a release agent, a charge control agent and so forth, and optionally making inorganic fine particles such as fine silica particles adhere or stick to the surfaces of the toner particles.
As colorants used in the black toner, black pigment particles such as carbon black and magnetite particles are used. The carbon black is relatively inexpensive and has a good blackness, but, because of its conductivity, the black toner tends to have low developing stability and charging stability. Also, when used in full-color image forming apparatus, because of the conductivity of the carbon black, the black toner causes image deterioration because of its difference in transfer performance from other color toners such as a cyan toner, a magenta toner and a yellow toner.
As for the magnetite particles, they are commonly used as a magnetic material of a magnetic toner used in a magnetic one-component developing system. Compared with the carbon black, the magnetite particles have a relatively large resistivity and can achieve relatively good developing stability and charging stability, but still have what are to be improved in their resistivity and coloring power. Especially where images are formed by a full-color system, the magnetite particles can not readily show electrostatically the same behaviour as pigments used in color toners do, because of the former's lower resistivity than the latter.
The magnetic toner also commonly contains the magnetite particles in a large quantity in order to attain a sufficient magnetic binding force. Hence, it is difficult for the toner to be improved in low-temperature fixing performance. Especially when used in full-color image forming apparatus, the fixing performance may come to differ between the magnetic toner and other color toners such as a cyan toner, a magenta toner and a yellow toner, so that the gloss of images tends to be non-uniform.
Moreover, the magnetic toner used in a magnetic one-component developing system promises stable development, but still has an insufficient charging performance. Accordingly, when, e.g., used in combination with color toners of non-magnetic two-component developing systems, image deterioration due to the difference in transfer performance tends to come about because of the difference in charging performance like the case in which the carbon black is used. Also, where the magnetic toner is used in the two-component developing method, the magnetic toner, though improved in charging performance, tends to adhere magnetically to a developer carrying member such as a developing roller, tending to cause image deterioration during running.
In order to resolve these problems, toners are proposed which make use of a non-magnetic or weakly magnetic black pigment (e.g., Japanese Patent Applications Laid-open No. 2002-221821, No. 2004-102154, No. H03-220566 and No. 2004-054094). These toners have a small saturation magnetization, and are also usable in the two-component developing system. However, according to studies made by the present inventors, these toners have black pigment particles having an insufficient blackness, and hence it has been found unable to represent black characters or letters and fine lines having a sufficiently high reflection density and a high quality level. Also where the black pigment itself has a sufficient blackness to a certain degree, there have been a problem that no sufficient image density is achievable when the black toner is used alone and a problem that dark-red images are formed, if the black pigment is dispersed in a binder resin.
A proposal is also made on a toner which contains a low-magnetized metal oxide and has prescribed the color of fixed images (Japanese Patent Application Laid-open No. 2002-196528). However, the metal oxide in the above has a reddish black color, and has substantially been not usable in black toners unless a blue pigment is used in combination. A proposal is also made in regard to iron-titanium composite oxide (e.g., Japanese Patent Application Laid-open No. H03-002276). There, however, has been a problem that, when the composite oxide is low-magnetized, image density is achievable to a certain extent at solid image areas but images are reddish at halftone areas. A black pigment powder is also presented which makes use of iron-titanium composite oxide and has a saturation magnetization of from 5 to 40 Am2/kg (e.g., Japanese Patent Application Laid-open No. 20004-161608. However, a problem has tended to arise during running when a black toner containing such a black pigment powder is used to perform two-component development or used in combination with color toners to form full-color images.